Activities of small synthetic peptides and partially purified fractions of hypothalamic extracts have been determined. Evidence for a factor that released FSH and LH has been obtained from partially purified bypothalamic extracts that may be FSH-RS since it can be chemically separated from LH-RH, and furthermore, does not cross react in the RIA for LH-RH. Hypothetical models of the putative prohormones TRH and LH-RH have been synthesized and their activities determined in incubates containing hypothalami plus pituitaries or pituitiaries alone. Results of these studies support the possibility that TRH and LH-RH may be first synthesized as prohormones and that there may be separate and distinct converting enzymes for "proTRH" and "ProLH-RH". In vitro studies have been performed to investigate the regulation of GH release from the pituitary of rats using the synthetic pentapeptides, Tyr-DT0rp-Gly-Phe-Met-NH2 (DTrp2) or Tyr-Ala-DTrp-Phe-Met-NH2 (DTRP3) and somatostatin (SRIF). DTrp2/3 may be unique pentapeptides for learning more about the function and behavior of the putative GH-RH receptor of the pituitary somatotrophs and that it appears to be the dual effects of SRIF plus DTrp2/3 rather than SRIF alone that produces an irreversible inhibition of the stimulated release of GH at the putative GH-RH SRIF receptor(s). In vitro studies on the regulation of GH of the pituitary of immature and mature rats may reveal certain unique differences which could have important physiological implications in vivo.